This project is a descriptive study of the early development of cerebral cortex in fetal dogs. A new quantitative electron microscopic method has been devised to analyze the density of synapses as a function of cortical depth at successive stages of gestation. An analysis of the spatial distribution of synapses in somesthetic cortex during fetal life is being undertaken in order to delineate the location of early synapses. When synapse dense layers are located, then the post-synaptic elements can be identified by Golgi and E.M. studies. Attempts will be made to identify the pre-synaptic elements by injection of tritiated leucine into dorsal thalamus. The localization of synapses plus identification of pre- and post-synaptic elements will lead to a description of the early formation of neuronal circuitry. The distribution of synapses for each age also constitutes a parameter of normal development that is closely related to cortical function. Synaptic distributions from different ages will be compared to identify periods of rapid synapse formation. The relationship between synapse formation and dendrite growth is being studied. The ultrastructural features of the marginal layer of fetal dog cortex are being analyzed with particular attention to the maturation of Retzius-Cajal Cells. Surface evoked potentials are recorded from fetal somesthetic cortex. The waveform of evoked potentials will be related to the synaptic organization present at the same age.